1. Field of the Invention
The present invention relates to a color conversion apparatus for adjustment of color values, for use in a color copier apparatus, color image processing apparatus, etc. In particular, the invention relates to an apparatus for adjustment of color attribute values within specific regions of a reproduced color picture, while substantially preserving the color and lightness gradations of an original color picture.
2. Description of the Related Art
A color value, e.g. of a picture element in a color picture, can be expressed as a combination of attributes consisting of a hue value, a chroma (color saturation) value, and a luminance value (i.e. degree of lightness, on a scale extending between black and white). It is possible to process the respective color values of the picture elements of a color picture such as to modify only one color attribute of each picture element without altering the chroma and luminance attribute value. In that way, a reproduced color picture can be obtained in which, for example only a modification of hue has been achieved, without altering the tonal gradations of the original color picture. One prior art proposal for a color conversion apparatus to execute such alteration of color values in a reproduced color picture without changing the tonal gradations is described in Japanese Patent Laid-open No. 2-87777. With that apparatus, as shown in FIG. 1, successive R, G, B color values which are read by a scanner from an original color picture are converted into modified values, for use by a color printer. A color system conversion circuit 141 converts each R, G, B (i.e. combination of red, green and blue coordinates) color value into a corresponding combination of L*, a* and b* attributes, corresponding to a point in a L* a* b* color space. Of these, the L* attribute represents the luminance of the color value, while a* and b* specify the chromaticity, i.e. combination of hue and chroma attributes. The luminance attribute L* is transferred through a luminance conversion circuit 144 in which it is modified by a factor .gamma., to execute gamma-correction and obtain a corrected luminance attribute L*O. (a*1 b*1) represent the chromaticity of a color value from which color conversion is to be executed. Such a color value will be referred to in the following as a conversion origin color value. (a*2 b*2) represent a chromaticity to which conversion is to be executed. The operation is as follows. When an input (R, G, B) color value has a chromaticity (a* b*) that is identical to (a*2 b*2), i.e. so that identical inputs A and B are supplied to the judgment circuit 142, then a logic "1" level output signal is produced from the circuit 142 and supplied through an OR gate 145 to the control input terminal of a selector circuit 143. In that case, the selector circuit 143 transfers the chromaticity (a*1 b*1) to be outputted as the final chromaticity value (a*0 b*0). If however the chromaticity of the input color value is different from (a*2 b*2), so that a "0" output is produced from the judgement circuit 142, then the selector circuit 143 transfers the chromaticity (a* b*) of the input color value without change to become the final chromaticity value. In that way, color conversion will be executed only for one specified color (i.e. combination of hue and chroma), appearing anywhere in the original color picture, into a desired color (combination of hue and chroma).
If the entire reproduced picture is to be printed in monocolor, with the chromaticity (a*1 b*1), then the "monocolor" input signal to the OR gate, which is normally at the "0" level, is fixed at the "1" level. The (a*1 b*1) chromaticity is thereby continuously transferred through the selector 143 in place of the chromaticity of each of the input color values, so that the entire reproduced color picture will appear in monocolor, with the (a*1 b*1) chromaticity.
However, with such a prior art apparatus, the following problems arise:
(1) Due to the fact that only color information is used to specify the color conversion processing that is to be executed, all of the portions of the image which have the same color as a specified value will be converted to the same new color, irrespective of position within the color picture. That is to say, it is not possible to selectively change the color of one or more specific regions of the picture. PA0 (2) Due to the fact that conversion is limited to a single color, it is impossible to change a plurality of specific regions within the color picture to respectively different colors. PA0 (3) In the color system conversion circuit 141, conversion of each (R, G, B) color value to (L* a* b*) is executed by computing, as each set of R, G and B values for a picture element receives (i.e. from an image scanner aparatus) the corresponding L*, a* and b* values for that picture element. However in general, the operation for converting a color value from (R, G, B) coordinates to (L* a* b*) coordinates is non-linear, so that it is not possible to obtain the (L* a* b*) coordinates by a single mathematical operation such as a matrix operation. It is first necessary to execute matrix multiplication operation to obtain from the (R, G, B) coordinates of the picture element a set of values designated as X, Y and Z, then obtain the values of L*, a* and b* for that picture element by three computations using respective non-linear equations. It is then necessary to convert the set of (L* a* b*) coordinates for the picture element into a set of color-compensated printing signal values, i.e. (Y (yellow), M (magenta), C (cyan), B (black)) values, which can be supplied to a color printer section. That conversion is executed by a section which is not shown in FIG. 1.
Thus with such a method, due to the complex computations which must be executed for each picture element color value in order to obtain signal values to be supplied to a color printer section, and since these computations must be executed by real-time operations to convert the (R, G, B) coordinates of each input color value in parallel into (L* a* b*) coordinates, the circuit scale must be large. Moreover, when such a method is applied to a color copier apparatus of the type in which picture-sequential printing is used, it is necessary to repeat these computations successively for each of the picture elements, during each operation of printing one color of the reproduced picture. The term picture-sequential printing as used herein signifies a method of color printing in which one color component is printed for the entire picture, then the next color component for the entire picture, and so on. For example, the C (cyan) color values for all of the picture elements of the picture might be printed first, then the M values for the entire picture, then all of the Y values, then finally all of the BK values.